The overall research goal is the determination of the structure, mechanism of action and role in the cell of phosphoryl transferring enzymes. Porcine adenylate kinase continues to be the principal subject for study because an atomic model from X-ray diffraction studies is available and because so much still needs to be learned about the nature of the active site and the mechanism by which a phosphoryl group is transferred. A related enzyme, GTP-AMP phosphotransferase, from the matrix of beef heart mitochondria has been purified and crystallized. This enzyme is involved in a step of substrate level phosphorylation in the mitochondrial matrix. Physicochemical properties of the enzyme are being determined.